1. Field of the Invention
This invention relates to a rolling bearing to be used for automobiles, agricultural machines, construction machines and steel machines, etc., particularly to a rolling bearing for use in a transmission or engine.
2. Related Background Art
As is well known in the art, the lubricant oil of a rolling bearing is subject to entrainment of foreign substances such as cut powder, dreg, flash, abraded powder, etc. Such substances may cause damage to the rolling members and inner and outer races of the bearing, and thus shorten the life of the bearing. The life of the bearing may be shortened in some cases even to about one tenth as compared with the case when there is substantially no entrainment of foreign substance in the lubricant oil.
For solving this problem, the following improvements have been proposed in the prior art.
(a) Increased hardness of the inner and outer races and rolling members. For example, SUJ2 bearing steel may be subject to hardening in brine to enhance the hardness of the bearing to a Rockwell hardness H.sub.R C of 64 or higher.
(b) Make carburized hardened layer of inner and outer races and rolling members deeper. For example, a carbon steel with a carbon content of about 0.5 wt.% or less such as SCR 420, 430, and SAE 4320 and 4340 may be subjected to carburizing and heating treatment for a long time. The intended result is to improve bearing life by increasing resistance to formation of impressions on the race surfaces and on the rolling member contact surfaces.
(c) Enhance crack toughness of the material.
For example, temperature controlled heating treatment such as martempering, etc. may be applied to a bearing steel such as SUJ3 or SUJ5. Enhanced crack toughness is intended to retard progress of cracks which form at surface impressions.
The measures which have been used in the prior art as described above involve the following problems which render them inadequate.
First, concerning (a), the high hardness of inner and outer races and rolling members results in poor toughness of the race surfaces and the surfaces of the rolling members. Thus, even though foreign substances may only form very slight impressions, cracks form at any early stage at the damage sites, and flaking (peel-off) is liable to occur through propagation of cracks. Consequently, the life of the bearing is not improved.
Concerning measures (b) and (c), deeper carburizing hardening requires longer hardening time, while the use of special temperature-controlled heating treatment reduces heating treatment productivity. Further, even with such heating treatment, sufficient bearing life is still not obtained.